Document upperXYZ functions.

[johnnymatthews]

Create documentation for the upperXYZ function. See #1833 for details on how to complete this issue.

• upper
• upperUTF8

Child issue of #1833.

[vdimir]

Converts string to UPPER-CASE

Note from the code:

/** If the string contains UTF-8 encoded text, convert it to the lower (upper) case.
  * Note: It is assumed that after the character is converted to another case,
  *  the length of its multibyte sequence in UTF-8 does not change.
  * Otherwise, the behavior is undefined.
  * ```